If you can't see the source of the leak looking down from the top, then you will have to get a close look from underneath. Get the vehicle SAFELY up on ramps, crawl under with a good light source, and have a close look. If the leak isn't obvious, wipe everything clean with rags, and try again. A big leak is not likely to be from the oil pan, but it's also not impossible if the gasket is badly shot.